Pulmonary surfactant is a complex mixture of lipids and proteins that promotes the formation of a monolayer at the alveolar air-water interface and, by reducing the surface tension, prevents the collapse of the alveolus during expiration. Premature infants, and occasionally full term neonates, sometimes suffer from a condition known as respiratory distress syndrome (RDS) due to the lack of sufficient endogenous pulmonary surfactant. Artificial pulmonary surfactants have therefore been developed to treat this condition thereby reducing infant morbidity and mortality. Likewise, artificial pulmonary surfactants have also been indicated in the treatment of adult respiratory distress syndrome.
One of these artificial pulmonary surfactants, known as KL4, is disclosed in U.S. Pat. Nos. 5,164,369 and 5,260,273. As described therein, KL4 is a synthetic pulmonary surfactant composition comprising a pharmaceutically acceptable phospholipid admixed with a polypeptide having alternating hydrophobic and positively charged amino acid residues. As formulated for clinical use, the composition is a liposome comprised of dipalmitoyl-phosphatidylcholine (DPPC), palmitoyloleoylphosphatidylglycerol (POPG), palmitic acid (PA) and the synthetic peptide KL4 suspended in a buffered aqueous medium. The final drug product is a liposomal suspension intended for direct instillation into the lung.
As mentioned, the artificial pulmonary surfactant, KL4, is a liposomal formulation. Liposomes are small vesicles comprising amphipathic lipids arranged in spherical bilayers. Liposomes may contain many concentric lipid bilayers separated by aqueous channels (multilamellar vesicles or MLVS), or alternatively, they may contain a single membrane bilayer (unilamellar vesicles), which may be small unilamellar vesicles (SUVs) or large unilamellar vesicles (LUVs). The lipid bilayer is composed of two lipid monolayers having a hydrophobic "tail" region and a hydrophilic "head" region. In the membrane bilayer, the hydrophobic "tails" of the lipid monolayers orient towards the center of the bilayer, whereas the hydrophilic "heads" orient toward the aqueous phase.
Liposomes may be used to encapsulate a variety of materials by trapping hydrophilic compounds in the aqueous interior or between bilayers, or by trapping hydrophobic compounds within the bilayer. As such, they are particularly useful to deliver biologically active materials by encapsulating compounds which exhibit poor aqueous solubility or which exhibit unacceptable toxicity at therapeutic dosages.
Currently, the KL4 liposomal pulmonary surfactant composition is prepared and stored in a liquid state. Because the peptide and phospholipid components of the composition are subject to degradation by hydrolysis in the aqueous liquid state, the solution must be kept under refrigerated conditions to retard the hydrolysis and achieve long term stability. However, refrigeration is a drawback to commercial application of the product. Accordingly, the main objective of the project culminating in the instant invention was to provide a KL4 pulmonary surfactant dosage form with improved stability at room temperatures.
It is known in the art that lyophilizing a product which is relatively unstable in aqueous solution can result in a product that is stabilized and therefore has a longer shelf life than an aqueous solution. (See "Remington's Pharmaceutical Sciences", 15th Ed. Mack Publishing Co., Easton, Pa., pp 1483-1485). Accordingly, the technique known as lyophilization is often employed for injectable pharmaceuticals which exhibit poor stability in aqueous solution. This process involves freeze-drying, whereby ice is sublimed from frozen solutions leaving only the solid, dried components of the original liquid. The process has numerous advantages in that the aqueous solution can be processed and filled into dosage containers in a liquid state, dried at low temperatures thereby eliminating adverse thermal effects, and stored in the dried state where it may be more stable. In addition, the lyophilized product is ordinarily rapidly soluble and is easily reconstituted prior to administration to a patient. The lyophilization process has been applied to aqueous liposomal suspensions as well as ordinary liquid solutions.
Pharmaceuticals to be freeze dried are usually in aqueous solution ranging from 0.01 to 40% w/v in concentration of total solids. Final moisture content of the dried product is generally below 2% w/v, although some products may have a higher moisture content.
Thus, the object of the present invention is to provide a KL4 pulmonary surfactant composition with enhanced stability by the application of lyophilization to the liposomal suspension. Quite unexpectedly, it was discovered that the unique solid composition resulting from the lyophilization process exhibits improved viscosity characteristics when reconstituted.